Conventionally, there is known a plasma etching method (for example, Patent Document 1). In this plasma etching method, a processing gas containing fluorocarbon is introduced into a processing vessel in which a processing target object is provided, and a first high frequency power for exciting the processing gas into plasma and a second high frequency power for attracting ions to the processing target object are applied between electrodes within the processing vessel. The second high frequency power has a frequency lower than that of the first high frequency power, and the processing target object is etched by the generated plasma.
Patent Document 1: Japanese Patent No. 3,681,533
When forming a deep hole or groove by plasma etching (comparative example: FIG. 6A), however, twisting of a hole H is observed. In the comparative example, a two-step etching process is performed by using hexafluoro-1,3-butadiene (C4F6), difluoromethane (CH2F2), oxygen (O2) and carbon monoxide (CO) as a processing gas. In a first process, while applying the first high frequency power to an electrode, the second high frequency power is applied to the electrode while being switched ON and OFF cyclically. In a second process, while applying the first high frequency power to the electrode, the second high frequency power is applied to the electrode while being maintained ON continuously. In the respective processes, the ON/OFF switching of the first high frequency power is synchronized with the ON/OFF switching of the second high frequency power.